Optoelectronics with Carbon Nanotubes

Chapter IV Narrow-Band Electroluminescence from a Single Carbon-Nanotube p-n
Diode
1. Introduction
In Chapter III, we have reported on the characteristics of EL from single-tube CNTFETs
with a single, global back gate in the unipolar, high-bias regime. As we have seen, all
electrically-driven CNT light-emitters with a single gate are operated under highly non-
equilibrium carrier and phonon distributions. In those devices, high electric fields and currents
are either an essential requirement for the light generation process 71, 85, 87, 97, 117 , or an undesired
side effect 73, 76 , leading to high power requirements and low efficiencies. It is difficult to
determine the quantum efficiency of unipolar devices that operate by impact excitation since we
cannot determine the exciton generation rate, but the operative efficiency, defined as the number
of photons emitted per injected carrier, is comparable to that of conventional ambipolar
CNTFETs, i.e., 10 -6 to 10 -7 . As a result of large applied power, the EL linewidths are broad (80–
300 meV, including the results presented in the previous chapter and in literature elsewhere),
obscuring the contributions of the individual optical transitions. One way to overcome the above
shortcomings is to use a p-n junction to create photons efficiently in a low electric field. In this
chapter, we discuss electrically-induced light emission from individual CNT p-n diodes.
P-n junction diodes are fundamental to today’s optoelectronic devices, such as photo-
detectors, light emitting diodes (LEDs), and lasers. The demonstration of light emission from
CNT p-n diodes is thus a fundamental step towards a possible technological use of CNTs as
nanometer-scale light sources. A realization of CNT p-n junction was first achieved by Lee et al.
as early as 2004 using embedded split-gate structure 80 . As illustrated in Figure IV-1 (a), two
separate gate electrodes that couple to two different regions of a SWNT are used. One gate is
biased with a negative voltage, drawing holes into the channel, and the other gate is biased with a
positive voltage, resulting in an accumulation of electrons in that region of the channel. In this
way, a p-n junction can be formed between the two and the devices behave very much like
conventional semiconductor diodes. Indeed, Lee et al. showed that an ideal p-n junction
transport can be achieved with CNT p-n diodes 80, 81 .
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